Restraining the widespread use of solar cells and windmills for power generation is the problem that energy demand does not vary in the manner of the availability of sunlight and wind. Therefore, an efficient means of storing and releasing energy during periods of excess and insufficient generation by these sources is needed for the promotion of these natural renewable energy sources. Coupling an unreliable primary generator to a regenerative fuel cell can perform that function. The regenerative fuel cell operates by storing energy by the generation of hydrogen via the electrolysis of water and releasing energy upon demand by oxidation of the hydrogen, generally regenerating water. By using one or more fuel cells matched in capacity to the primary power generator, a reliable system can result. Fuel cells are attractive because they can be compact and can possess a low internal resistance.
Present cells, capable of performing this function, generally rely on the use of platinum as an electrocatalyst. Unfortunately, platinum is of insufficient supply to make this a viable option for large scale application of the technology. State of art electrodes use precious metal catalysts for the generation and oxidation of hydrogen since non-noble metal electrocatalysts exhibit corrosion in acidic or other environments under which these reactions are carried out. Additionally, noble metals electrodes often exhibit performance degradation with time due to the loss of electrochemical surface area because the finely divided particles agglomerate.
An article entitled: “From Hydrogenases to Noble Metal-Free catalytic nanomaterials for H2 Production and Uptake”, Le Goff et al. Science, 326, 1384 (2009), reports that hydrogen (H2) generation results from water electrolysis and hydrogen oxidation over a nickel complex based electrocatalyst supported on multiwall carbon nanotubes (MWNTs). Control experiments, reported therein, indicate that MWNTs do not function as an effective electrocatalyst for either hydrogen generation or oxidation, and catalytic currents observed for the subject electrocatalyst during H2 oxidation and generation can be attributed solely to the Nickel complex supported by the MWNTs.
Other alternates to Pt have been examined as electrocatalysts for the formation of H2. Yang et al., Synthetic Metals 154, 69 (2005) entitled: “Hydrogen Generation using PPy-FMS modified PVDF Membrane and Other Substrates,” reports the polypyrrole containing catalytic ferrocene centers can increase electrocatalyst's conversion current compared to Pt. Unfortunately, it was concluded that a Pt supporting electrode is required for observation of long-term stability as deposition of the polypyrrole on stainless steel results in catalytic properties vanishing after just a few hours. The reverse reaction, H2 oxidation, is not disclosed in Yang et al.
Winther-Jensen et al., Adv. Mater. 22, 1727 (2010) entitled: “Conducting Polymer Composite Materials for Hydrogen Generation,” reports hydrogen generation from aqueous acidic electrolyte solution using a conducting polymer composite, comprising poly(3,4-ethylenedioxythiophene) (PEDOT) polymerized on a polytetrafluoroethylene (PTFE) membrane in the presence of polyethylene glycol (PEG), as the electrocatalyst. Catalytic activity of the PEDOT-PEG composite improved after 24 hour immersion in 1M sulfuric acid with the increased activity attributed to the swelling of the composite by that electrolyte solution. The overpotential observed for the composite electrode was higher than that observed with Pt. The reverse reaction, H2 oxidation, is not reported in Winther-Jensen et al.
Carbon has been extensively studied as a catalyst support in the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR). Although carbon appears to synergistically enhance the activity of the metallic catalysts for HER and HOR, carbon electrodes that are free of metallic catalysts are reported to promote HER only at a high overpotential but are not reported to promote HOR. Prosini et al., J. Power Sources 118, 265-269 (2003) disclose that carbon nanotube films produce hydrogen but required a significant overpotential for hydrogen oxidation. Misra et al., ACS Nano 3, 3903-3908 (2009) disclose that MWNTs displayed hydrogen evolution at a voltage of −10 V and is silent on HOR.
As stated in Kinoshita, K., Carbon: Electrochemical and Physicochemical Properties (Wiley, New York, 1988): “The hydrogen overpotential on most graphite and carbon surfaces is high; consequently these materials, by themselves, are not useful electrodes for hydrogen oxidation/evolution.”; and “ . . . carbon does not have electrochemical activity for the electrochemical oxidation of H2.” To this end, an electrode comprising an effective non-noble metal, stable electrocatalyst that exhibits little or no overpotential during the generation or oxidation of H2 remains a goal.